The invention relates to the general field of assembling together two parts of a structure in which one of the parts is made of composite material with fiber reinforcement obtained from a fiber preform made by three-dimensional weaving and densified with a matrix.
Non-limiting examples of applications of the invention include in particular assembling a metal strip on a leading edge of a turbojet blade made of composite material, and assembling a shell on a spar-forming structural core made of composite material for a turboprop propeller blade.
It is known to make a turbojet blade out of composite material by using fiber reinforcement obtained from a fiber preform made by three-dimensional weaving and densified with a matrix. Compared with other known techniques for fabricating a composite material blade, making a fiber preform by three-dimensional weaving presents numerous advantages, in particular such as that of not needing any recourse to inserts or fitting any other separate element. Reference made by made to Document EP 1 526 285, which describes a method of fabricating such a fan blade.
Furthermore, it is known to fit a metal strip (or reinforcement) on the leading edge of such a composite material turbojet blade in order to protect the composite structure from abrasion/erosion and also in the event of an impact against a foreign body. This applies in particular for the fan blades of a turbojet, which are exposed to ingesting a bird, hail, ice, etc.
Typically, the metal strip, which by way of example is made by mechanical techniques such as stamping, forming, or electroforming, is adhesively bonded on the leading edge of the composite material blade by means of a bead of adhesive. That operation can be performed in a mold used for bonding the metal strip, or in a stove in order to cure the bead of adhesive that is applied on the strip, if any.
Assembling metal strip on the leading edge of a composite material blade in that way presents numerous drawbacks. In particular, when the blade is subjected to deformation in its chord direction or in its length direction, the local warping of the unit subjects the bead of adhesive to significant traction and tearing forces, which can cause the flanges of the metal strip to become unstuck. Unfortunately, once those flanges have become unstuck, the inertia of the metal strip under the effect of centrifugal force causes the strip to be ejected outwards.
Providing the strip with local reinforcement can then constitute a solution for limiting propagation of any unsticking, but the solutions that have been proposed, such as drilling and machining the airfoil and the strip in order to pass a mechanical fastener through them, give rise to an additional operation and damage the airfoil locally. Furthermore, the position zones of the strip need to be highly aerodynamic, which requires the means for assembling the strip to fit as closely as possible to the shape of the strip. Unfortunately, with fan blades having shapes that are ever more complex, this constraint becomes difficult to satisfy when using the assembly solutions of the prior art.